Spectrum-adaptive networking

ABSTRACT

The present invention increases the available spectrum in a wireless network by sharing existing allocated (and in-use) portions of the RF spectrum in a manner that will minimize the probability of interfering with existing legacy users. The invention provides interference temperature-adaptive waveforms, and a variety of physical and media access control protocols for generating waveforms based on measurement and characterization of the local spectrum. The invention measures the local spectrum at a receiving node, generates an optimal waveform profile specifying transmission parameters that will water-fill unused spectrum up to an interference limit without causing harmful interference to primary and legacy transmitters using the same frequency bands, and enables simultaneous transmit and receive modes at a multiplicity of transceivers in a wireless network. The invention also provides closed loop feedback control between nodes, co-site interference management, intersymbol interference mitigation, wide sense stationary baseband signaling and modulation, and power limited signaling for avoiding detection and interception.

RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.10/689,763, filed Oct. 22, 2003, which claims priority under 35 U.S.C.§119 to provisional Application No. 60/420,930, filed Oct. 24, 2002, theentirety of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to radio frequency spectrummanagement systems, and, more particularly, to systems and methods forrecovering and/or sharing radio frequency spectrum in radio frequencybands already populated by legacy users (such as cell phone users) andhigh-priority users (such as public safety, military and governmententities) without exposing those legacy and high-priority users toharmful interference.

2. Description of the Related Art

Wireless communications network bandwidth continues to shrink at analarming rate. Increasing demand for spectrum-based services and devicesis putting a strain on long-standing and out-moded spectrum allocationand use policies. Managing interference levels among the rapidlyincreasing number of users has become extremely difficult because of thegreater density, mobility and variability of “next generation” (XG)radio frequency emitters.

Current spectrum management policy seeks to assign locally unoccupiedportions of the RF spectrum to XG users. The Federal CommunicationsCommission (“FCC”) Spectrum Management Policy Task Force has recommendedadoption of a policy of “Interference Protection,” which defines anacceptable level of interference to primary users from secondary usersin terms of this interference temperature. Under the recommended policy,secondary users of a band are required to accept interference fromprimary users, and must cause no “harmful” interference to the primaryusers. Thus, the policy permits secondary (e.g., unlicensed) users toradiate only enough power in an area of interest to raise theinterference temperature in the band to a specified threshold T₀ for theband, service, and locality. The receivers of the primary (e.g.,licensed) users are then expected to tolerate this specified level ofinterference. This proposed arrangement, if it is widely adopted, willcreate an opportunity to “underlay” existing primary applications withlow-power, low-impact opportunistic applications that operate below thethreshold.

While the Spectrum Policy Task Force Report recommends a new set ofrules for spectrum use that in turn will provide a sound framework forusing the available spectrum more efficiently, the report does notaddress many important and heretofore unanswered questions about how tobuild and configure networks and devices that comply with the new set ofrules. Accordingly, what is needed are tools, devices and applicationsXG users can build, configure and deploy in order to take advantage ofthe proposed spectrum policies.

SUMMARY OF THE INVENTION

The present invention provides key enabling technology to implement theFCC's new flexible spectrum use policy. The invention addresses bothindividual spectrum management devices and provides an integrated systemconcept for dynamic, adaptive, radio frequency spectrum assignment anduse. The result is far greater spectrum efficiency, providingmegabit/sec rate communications networks that can extend far beyond thecapabilities of existing wireless networking systems and devices. Theinvention provides a way to underlay new services on existing bandwidthallocations with minimal or no interference to, and from, existinglegacy users, while providing up to 30 times greater throughput than thecurrent systems.

To accomplish these goals, a node of a network communications systemconfigured to operate in accordance with the present invention isconfigured to:

-   -   Continuously carry out real-time sensing and characterization of        the local spectrum usage by (potentially interfering) narrowband        and wideband emitters;    -   Dynamically and autonomously adapt (on a time scale of        milliseconds) to the local spectrum environment by selecting and        controlling the waveforms (power spectral density (PSD) and        Media Access Control (MAC) protocols) that its network neighbors        use when transmitting to this node;    -   Automatically carry out a closed loop power control algorithm        with each neighbor to throttle back on unnecessarily high power        levels, thereby enhancing Low Probability of Detection (LPD);    -   Apply transmission security (“TRANSEC”) parameters to the spread        spectrum modulation process in order to enhance Low Probability        of Intercept (LPI); and    -   Carry out packet forwarding (routing) in a way that balances        aggregate network throughput against average end-to-end delay.        (This results in real time traffic, e.g., voice, being sent with        higher power, minimizing latency due to channel access delays at        multiple hops, and bulk traffic being sent with lower power,        minimizing network self-interference, maximizing spatial reuse        of frequencies and enhancing LPI/LPD).

As described in detail below, the present invention also provides ahighly advanced networking communications architecture for implementingthe policies recommended by the FCC Task Force. The design of thearchitecture combines dynamic spectrum management techniques withmatching adaptive networking and full exploitation of multipletransceivers per communications node. The invention also provides theflexibility and scalability, and may be easily adapted for use withother forward-looking wireless communications systems and technologies.

The present invention solves the “spectrum crisis” currently plaguingcommercial, military, government and private users by providing a way tounderlay spectrum-efficient megabit rate networking onto bands allocatedfor other purposes. The invention can thus be used, for example, tounderlay military networking below any narrowband-channelized spectrumwhere individual channels have less than 100% duty cycle, such as incommercial cellular, without interfering with existing legacy users ofthese bands. Initial use of existing frequency allocations ensures thatthe invention may be implemented domestically without displacing orupsetting existing users. In overseas locations and in wartime, however,the invention is not restricted to these particular bands, and theflexible hardware and software made possible by the invention will alsooperate in other frequencies without hardware modification.

In general, the present invention provides a method for managinginterference in a radio communications network, comprising the steps of:(1) receiving an aggregated radio signal at a first node in the radiocommunications network on a plurality of frequencies; (2) determining apower level for the aggregated radio signal for each frequency in theplurality frequencies; (3) subtracting the power level at each frequencyfrom a power limit to produce a power differential for each frequency;and (4) instructing other nodes in the radio communications network toavoid using a transmission frequency corresponding to a non-positivepower differential in the plurality of power differentials to transmitto the first node. A government agency, such as the FCC, an industrystandards group, or other rule-making body may specify the power limit.

In preferred embodiments, the method further comprises the steps of: (5)receiving a transmission from a second node in the radio communicationsnetwork; and (6) discarding any portion of the transmission carried onthe transmission frequency the second node was instructed to avoid.Typically, although not necessarily, the discarding step is accomplishedby applying an optimal matched filter to the transmission, said optimalmatched filter being keyed to the instruction sent to the second node.By discarding a portion of the received transmission in this manner (theprocess is called receiver excision), interference produced by otheremitters (e.g., legacy primary users) in the wireless environment isfiltered out of the transmission.

The power level may be determined by acquiring a plurality ofinstantaneous power level measurements for each frequency andcalculating an average power level based on the plurality ofinstantaneous power level measurements. This creates a “model” powerlevel, of sorts, that may reflect a more accurate measure of the powerlevels present at the plurality of frequencies over a given period oftime. Alternatively, the model power level also may be determined bycalculating a median power level based on the plurality of instantaneouspower level measurements.

In preferred embodiments, the method also includes closed loop powercontrol. Each receiver sends to other nodes in the network a request toadjust (or limit) its transmission power level on frequenciescorresponding to positive power differentials so that the receiverreceives transmissions from all of its neighbors at the same powerlevel. This feedback from each receiver in the network minimizes thetransmit power on reliable links, maximizes the ability to achievespatial separation among transmissions on the same frequency, andthereby minimizes network self-interference. Closed loop power controlfunctionality also improves low probability of intercept (LPI) and lowprobability of detection (LPD), and minimizes battery drain for mobileand/or portable transmitting nodes.

In preferred embodiments of the invention, the aggregated signal iscontinuously monitored and the measured power level is continuouslyupdated to account for the fact that some or all of the nodes may bemobile, and therefore constantly moving around, and new emitters(causing increased levels of interference) may come on line.Accordingly, each receiver in the network is preferably configured toprovide constant, or at least fairly constant, updated instructions toits neighbors setting forth the optimal transmission parameters to useto transmit data to that receiver.

The optimal transmission parameters sent out to neighboring nodes by areceiver node may be embodied in an optimal waveform profile, whichitself may contain information and instructions beyond power spectraldensity values. Such profile might also contain, for example, a scheduleof optimal times to transmit information to the receiving node.

In some embodiments, it may be advantageous, desirable or necessary tocompress the optimal waveform profile prior to transmitting it to aneighboring node. It may also be advantageous to specify a particularwaveform pattern so that the receiving node will be able to determinefrom which node the waveform came.

In another aspect of the invention, another method for managinginterference in a radio communications network, is provided, comprisingthe steps of: (1) receiving at a first node in the radio communicationsnetwork an instruction transmitted from a second node to avoid using aplurality of frequencies to transmit to the second node; (2) filtering atransmission signal to remove power at frequencies that should beavoided; and (3) transmitting the filtered transmission signal to thesecond node. As before, the instruction may be embodied in or combinedwith an optimal waveform profile from the second node, the optimalwaveform profile being based on a plurality of power measurements for aplurality of frequencies, as measured from the second node, and thepower limit. In this aspect of the invention, the method may alsoinclude the step of decompressing the optimal waveform profile prior togenerating a transmission signal conforming to the profile.

In still another aspect, the invention provides a method of managinginterference in a radio communications network having a multiplicity ofnodes, each node in the multiplicity having attached thereto a set ofantennas oriented to face directions relative to other antennas attachedto the node. This method comprises the steps of: (1) dividing themultiplicity of nodes into a plurality of node clusters; (2) assigning aunique receiving frequency to each node in a node cluster; and (3)assigning a unique transmission frequency to each antenna in the set ofantennas attached to a first node in the node cluster; wherein theunique transmission frequency assigned to each antenna is equivalent tothe unique receiving frequency assigned to a neighboring node in thenode cluster.

In this aspect, the invention may further comprise transmitting outgoingmessages from the first node to the neighboring nodes using uniquereceiving frequencies assigned to the neighboring nodes. The first nodeis also capable of transmitting the outgoing message to the neighboringnode and receiving an incoming message from another neighboring nodesimultaneously.

In yet another aspect of the present invention, there is provided aradio communications device, comprising a receiver configured to receivean aggregated radio signal existing at the radio communications device,a spectrum analyzer, coupled to the receiver, configured to produce aseries of power readings for the aggregated radio signal for eachfrequency in a plurality of frequencies, a waveform profile generatorconfigured to produce a waveform profile based on the series of powerreadings and a power limit, and a filter, coupled to the receiver,configured to detect in the aggregate radio signal a transmission signaladdressed to the radio communications device, and to discard any portionof the transmission signal carried on a frequency corresponding to anunacceptable transmission frequency. The waveform profile defines theset of unacceptable transmission frequencies. A radio communicationsdevice operating in accordance with this aspect of the invention mayalso include a signal data processor configured to generate a modelpower level (e.g., an average or median power level) for the aggregatedradio signal for each frequency in the plurality frequencies based onthe series of power readings. The radio communications device typicallywould also include a transmitter configured to transmit the waveformprofile to a second radio communications device or to multiple radiocommunication devices in the network.

Preferably, the radio communications device in this embodiment of theinvention also includes a correlator, or a plurality of correlators,coupled to the filter, configured to determine whether the detectedtransmission signal contains a pattern uniquely associated with one ormore other radio communications devices in the network. Each pattern ina multiplicity of patterns is orthogonal to each other pattern so thatthe correlators may be used to identify discrete patterns being carriedby a single frequency or group of frequencies. The radio communicationsdevice may further include a media access controller (“MAC controller”)configured to toggle the radio communications device back and forthbetween a transmit mode and a receive mode.

In still another aspect of the invention, there is provided a method formanaging co-site interference in a wireless network, comprising thesteps of: (1) identifying a subset of nodes within a multiplicity ofnodes, each node in the subset being capable of transmitting data toeach other node in the subset in accordance with a defined connectivitythreshold and using a power setting that falls within a low power range;(2) defining a collection of transmission frequencies to be used bynodes of the subset only when transmitting to a node outside of thesubset; and (3) permitting only one node of the subset at a time totransmit using any transmission frequency within the collection. In thisembodiment, access to certain frequencies for data transmissions areserially allocated to the members of the subset so that only one nodemay use those frequencies at a time. Since nodes in a mobile network maybe moving continuously, the method may further include the step ofupdating the subset of nodes in short (i.e., low power) range of eachother according to a schedule, at regular intervals, when it isdetermined that a significant amount of node movement has occurred, orupon a determination that some combination of any one or all of theforegoing situations has occurred.

In preferred embodiments, the step of identifying a subset of nodes inshort range of each other is carried out using a K-Means vectorquantization algorithm. The step of serially allocating permission touse the long-range links is carried out using a point coordinationfunction.

In this embodiment, all nodes in the subset are configured to receivetransmissions on the any transmission frequency in the collection whileno node in the subset is transmitting on that transmission frequency.Preferably, a unique spread-spectrum code is associated with the subsetof nodes, so that transmissions from the subset may be identified assuch.

To better manage interference, another layer of clustering may beimplemented, which involves identifying a second subset of nodes withinthe multiplicity of nodes, wherein each node in the second subset iscapable of transmitting data to each other node in the second subset inaccordance with the defined connectivity threshold using a power settingthat falls within a medium power range. A second collection oftransmission frequencies is generated, this collection to be used by thenodes of the second subset to transmit using a power setting that fallsoutside the medium power range. Here again, only one node of the secondsubset at a time is allowed to transmit using any transmission frequencywithin the second collection.

Also provided is a method for managing congestion at an elevated node ina wireless network, comprising the steps of: (1) identifying a subset ofnodes within a multiplicity of nodes, each node in the subset beingcapable of transmitting data to each other node in the subset inaccordance with a defined connectivity threshold and using a powersetting that falls within a low power range; (2) defining a collectionof transmission frequencies to be used by nodes of the subset only whentransmitting to the elevated node; and (3) permitting only one node ofthe subset at a time to transmit to the elevated node using anytransmission frequency within the collection. This process manages thenetwork communications in such way as to avoid high fan-in congestion.In this embodiment, nodes of the subset are serially allocatedpermission to use certain frequencies to transmit to the elevated node.All nodes in the subset are configured to receive transmissions on theany transmission frequency in the collection while no node in the subsetis transmitting on the transmission frequency.

A wireless network configured to operate in accordance with the presentinvention comprises a plurality of short-range links, a multiplicity ofnodes configured to automatically identify a cluster of nodes within themultiplicity capable of transmitting data to each other node in thecluster via the plurality of short-range links using a power settingthat falls within a low power range. The multiplicity of nodes is alsoconfigured to self-select a node in the cluster to act as a long-rangetransmission manager, which permits only one member of the cluster at atime to transmit using any power setting that falls outside the lowpower range.

A radio communications device in accordance with this embodiment maycomprise a transmitter configured to send data to any other media accesscontroller in the multiplicity using a power setting that falls within alow power range, and a media access controller configured to receive aplurality of requests from the multiplicity of other radiocommunications devices to transmit using a power setting that fallswithin a high-power range. Here again, the media access controller isconfigured to grant only one request in the plurality of requests at atime. The radio communications device may be further configured toreceive any transmission having a power level that falls within thehigh-power range while no radio communications device in themultiplicity of other radio communications devices is transmitting usingthe power level. Preferably, the radio communications device alsoincludes a correlator, or multiple correlators, configured to determinewhether the transmission contains a pattern associated with a particularradio communications device in the multiplicity of radio communicationdevices.

Finally, a method of managing real-time data traffic in a wirelesscommunications network is provided. The network has a multiplicity ofnodes and is configured to transmit a data stream along a route from asource node in the network to a destination node in the networkaccording to a routing protocol. This method comprises the steps ofreceiving at an intermediate node in the route a data packet from thedata stream and a request to transmit the data packet to the next nodein the route and determining whether the next node is operating in areceiving mode. If the next node is operating in the receiving mode, thedata packet is transmitted to the next node. However, if the next nodeis not operating in the receiving mode, then the data packet isforwarded to any other node in the multiplicity of nodes that is both inthe receiving mode and nearer to the destination node than theintermediate node.

Aspects of the present invention may be implemented using software radiotechnology, which provides flexibility to make dynamic adaptive use ofspectrum under rapidly changing interference conditions. Software radiosalso support advanced adaptive Media Access Control (MAC) protocols.Vanu, Inc. (www.vanu.com) of Cambridge, Mass., for example, makessoftware radios suitable for using as a transceiver in a networkconfigured to operate in accordance with the present invention.

Software radio technology also enables implementing embodiments of thepresent invention using COTS (commercial-off-the-shelf) processor-basedradios that will improve over time, incorporating new signalingprotocols as they become available, and incorporating ever moreefficient components in terms of size, weight, and power. Softwareradios may be built around Intel Pentium 4 processors, for example,which currently operate at clock speeds of about 3.2 GHz, are projectedto operate at clock rates beyond 10 GHz by 2005. Because the softwareused in a software radio is portable, networks and networking devicesdesigned according to embodiments of the present invention can beupgraded over time to feature the latest and fastest COTS processors. Inaddition, the invention can easily be configured to use RF-to-digitalfront-end technology, which can provide all the necessary analogfunctions on a compact card. Rapid technology improvements in this areamake these front ends quite affordable, so there is no impediment tooperating each network node with multiple independent transceivers. Thisrapid evolution in transceiver technology is evident, for example, intoday's two-chip CMOS implementations of IEEE 802.11a 54 Mb/s OFDMtransceivers.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing summary, as well as the following detailed description,will be best understood in conjunction with the attached drawings, whichare incorporated in and constitute part of the specification. Thedrawings illustrate preferred embodiments of the invention, and,together with the description, serve to explain the principles of thepresent invention.

FIG. 1 depicts a high-level block diagram illustrating an arrangement ofsome of the physical components in a transceiver configured to operateaccording to an embodiment of the present invention.

FIG. 2 shows a reference model defining the characteristics of a channelbetween a transmitter and a receiver configured to operate in accordancewith principles of the present invention.

FIG. 3 graphically illustrates a method of calculating the powerspectral density (PSD) of an optimal waveform according to embodimentsof the invention.

FIGS. 4 through 11 contain plots and graphs illustrating the results ofspectrum measuring experiments supporting the need for and the benefitsof the present invention.

FIG. 12 depicts a model illustrating an integrated system conceptaccording to an embodiment of the present invention.

FIG. 13 illustrates one approach, in accordance with the principles ofthe present invention, to mitigating intersymbol interference.

FIG. 14 shows a high-level block diagram of a network node 1400 (such asa software radio, a router, etc.) configured to operate according toembodiments of the present invention.

FIG. 15 contains a high-level flow diagram illustrating the stepsperformed in an embodiment of the present invention in order to processsignals and manage interference received at a node, such as the nodeshown in FIG. 14.

FIGS. 16, 17 and 18 show the results of simulation experiments forcoloring (assigning different frequencies) a random layout of fiftynodes in a network.

FIG. 19 shows a high-level flow diagram illustrating the steps that maybe performed by a processor configured to assign receive frequencies toa multiplicity of nodes, in accordance with the present invention, toensure that there are no adjacent nodes that use the same receivefrequency.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to preferred embodiments of theinvention, examples of which are illustrated in the drawings. Notably,the present invention may be implemented using software, hardware, orany combination thereof, as would be apparent to those of ordinary skillin the art. Therefore, the figures and examples below are not meant tolimit the scope of the present invention or its embodiments orequivalents.

The invention provides dramatic improvements in assured wirelesscommunications by dynamic redistribution of allocated spectrum andadvanced Media Access Control (MAC) protocols. FIG. 1 shows anarchitecture that can be used to achieve these improvements according toan embodiment of the present invention. FIG. 1 shows the variousfunctions of the architecture as they are arranged in their appropriateprotocol layers. What follows is a description of each layer of thedesign.

General Architecture Description

The invention provides a system for using local, metropolitan, andwide-area network (LAN, MAN, and WAN) bands for high-speed(megabit-rate) networking and seamless interoperability with currentsystems. Taking advantage of military L-Band, UHF and VHF allocations(cross-banding within a single network), for example, as shown in Table1 below, different links are created for these different purposes.

Per-Hop Data Band Use Range Path Loss Rate Analogy Mil-3: W-LANs 0.2 Km 60-90 dB ~10   Star Trek 1710-1850 (Squad) Mb/s “Comm Badge” MHzMil-UHF: W-CANs   5 Km  90-120 dB ~2   Star Trek 225-400 (Company) Mb/s“Communicator” MHz Mil-VHF: W-MANs  30 Km 120-150 dB ~0.5 30 timesfaster 30-88 (Bde) Mb/s than SINCGARS MHz

This approach, namely providing a range of different link types, allintegrated into a single network with automatic cross-banding in nodeswith multiple independent transceivers, provides all the benefits neededfor successful megabit-rate wireless networking.

Path loss (PL) is the ratio of the signal power coming down the receivefeedline from an isotropic receive antenna, to the signal power going upthe transmit feedline to an isotropic transmit antenna. In the exampleof 90 dB path loss, every Watt transmitted turns into a nanoWattreceived. Assuming the use of isotropic antennas (gain=1=0 dB), the pathloss (PL) that can be tolerated on a link (link budget) is the ratio ofthe available transmit power, to the receiver sensitivity (requiredreceiver power). The Mil-VHF band provides much better propagation(longer range) for modest power, as the effective area of anomni-directional antenna may be represented by the equation λ 2/4π,where λ is the wavelength. This results in almost a 30 dB improvement inpath loss (P.L.), for 30 times the line of sight (LOS) range compared tothe Mil-3 band. Diffraction around wavelength-sized obstacles (e.g., 10meter hills or buildings) can make an even greater difference beyondLOS.

In contrast, the Mil-3 band provides much higher available bandwidth,permitting far higher data rates over short ranges. In fact, the shortrange here results in both high spatial reuse of the frequencyallocation, and a low power, LPI/LPD signal with wide spectrumspreading. Having an unobtrusive signal eliminates the need to contestspectrum “ownership”.

The Mil-UHF band is intermediate between these two in both range andavailable bandwidth, satisfying the need for medium range links.

As shown in FIG. 1, an architecture structured according to anembodiment of the present invention compromises both physical layer 104and media access control (MAC) layer 102 components. At the physicallayer 104 the functional components of the architecture may includecomponents for radio frequency modulation 110, interference temperaturemeasurements 120, narrow-band receive-transmit R-T excision 130,variable-gain spread spectrum 140 and closed loop link power control155. The MAC layer 102 comprises components for implementing co-siteclustering and serialized access to long-range links and elevated nodes162, opportunistic forwarding 164 and multiple correlators 166 fordetecting specific waveform patterns. Each of these components aredescribed in more detail below.

Co-Site Clustering and Multiple Correlators

A significant problem encountered when using the spectrum for wirelesscommunication is co-site interference, which occurs when transmissionsto distant nodes overwhelm reception by local nodes. The presentinvention addresses the co-site interference problem by using multipleindependent transceivers of different frequencies (bands) to establishshort links (e.g., 60-90 dB PL), medium links (e.g., 90-120 dB PL), andlong links (e.g., 120-150 dB PL), as shown in Table 1 above.

Co-site clustering and serialized access component 162 comprises MACprotocols that automatically define W-LAN groups (good internalconnectivity with short-range links), in which use of any medium-rangelink is serialized. W-MAN groups (nodes with good internal connectivityusing medium-range links) are defined, in which use of any long-rangelink is serialized.

A K-Means vector quantization algorithm may be used to define co-siteclusters and dynamically update them. Within each cluster (e.g., squadW-LAN), a point coordination function (PCF) is used to serialize accessto a medium-range link. Each cluster alternates between receiving overmultiple medium-range links, and transmitting over one medium-rangelink. Using multiple correlators 166 and a different spread spectrumcode for each W-LAN permits simultaneous use of medium-range links bydifferent squads without interference.

In a preferred embodiment, a number of nodes have elevated antennas toprovide long-range links for “Small World” latency reduction andlow-latency quality of service (QoS) traffic. As described below, aspecialized MAC protocol to handle the very high fan-in usuallyassociated with using elevated nodes may be used to avoid congestion atthe elevated node.

For extremely low delay traffic, packets are forwarded to any node inthe direction of the destination (lower hop count) that is not currentlytransmitting (like a soccer player passing to an open teammatedownfield). Although this violates the customary practice of strictprotocol layering, by using instantaneous information from the MAC layerto guide network forwarding decisions, it eliminates the dominantend-to-end latency effect of channel access delay at each hop.Opportunistic forwarding component 164 carries out this function. Theopportunistic forwarding component 164 carries out forwarding of packetsbased on their indicated (differential services, or diffserv) quality ofservice. Bulk traffic is forwarded via low-power multi-hop routing, tomaximize spatial reuse of frequencies and enhance LPI/LPD. Real-timevoice traffic traverses the network in fewer hops at higher power inorder to minimize end-to-end latency.

Closed Loop Link Power Control

Closed loop link power control component 155 provides feedback from eachreceiver to a neighboring transmitter to minimize transmit power for areliable link. This maximizes spatial reuse of frequencies, minimizesnetwork self-interference, improves LPI/LPD, and minimizes battery drainfor portable nodes.

This may be accomplished by keeping transmit power density low (e.g.,<50 mW/MHz or 1 W per 20 MHz) in any frequency or time interval, subjectto topology control to maintain a connected network. This also enhancesspectrum compatibility with other friendly narrowband and wideband (NBand WB) systems. This limit on power density also helps achieve LPI bythwarting linear (spectrum analyzer) interceptors. Wideband waveformsmay be used to achieve the desired ranges.

Variable-Gain Spread Spectrum

Variable gain spread spectrum component 140 controls the processing gain(spectrum spreading factor) associated with a node in the network. Inpreferred embodiments, different levels of security may be implementedby varying the processing gain (PG). In the military battle context, forexample, very high PG (e.g., 30 dB) may be used during covert insertionand special operations; medium PG (e.g., 20 dB, higher data rate) may beused for entry and positioning of major forces; and modest PG (e.g., 10dB, highest data rates) be used during actual battles. Flexible softwareradio technology enables the use of variable PG.

Multiple receive correlators 166 in the MAC layer 102 may be used toreceive simultaneously from multiple neighbors. Spread spectrumsignaling increases bandwidth by a factor of the PG. Using multiplecorrelators recoups spectrum efficiency by a factor of the number ofneighbors, up to the square root of the PG.

Narrowband R & T Excision

Narrow band R-T Excision component 130 controls the process for usingzero power at certain frequencies during transmission and ignoringinformation (power) existing at those frequencies on the receiving end.In preferred embodiments, orthogonal frequency division multiplexing(OFDM) waveforms with 25 KHz channels that support transmit and receivenarrow band excision may be used to eliminate interference with legacysystems. The OFDM signal (which is the sum of many complex exponentials,each with an independent phase) approximates a Gaussian envelopefeatureless waveform for LPI/LPD. R&T excision may be adjusted on a verydynamic basis (milliseconds) to underlay spectrum allocated for a legacychannelized narrow band system.

In preferred embodiments, short-term power spectrum measurements (<<1sec) may be taken at the receiver, and narrow band (25 KHz) excision ofspectral spikes are used to minimize interference produced by legacysystems. Matching narrow band excision of the same spikes at neighboringtransmitters eliminates interference experienced by narrow band legacysystems.

Interference Temperature Measurements

Interference temperature measurements component 120 monitors aggregatesignals present at the receiver in the wireless network. In preferredembodiments, each receiver does its own spectral analysis of localinterference, assigns its own receive frequencies automatically, andreports them to its neighbors over a common, low data rate,configuration channel. Selecting low-interference frequency rangesminimizes transmit power requirements and avoids congested spectrum.

Radio Frequency (RF)-to-Baseband, Using Multiple Transceivers

As described in more detail below, a wireless networking architectureaccording to the present invention may also include radio frequencymodulation component 110 in order to increase throughput capacity andprovide more secure transmissions. For example, and as described in moredetail below, multiple transceivers may be used at each node to supportsimultaneous operation in multiple bands, simultaneous spectral analysisand communications, and other functions. Wide-sense stationary waveformsmay also be used to thwart 2^(nd) order cyclo-stationary interceptors.Zero mean, equal power, uncorrelated I and Q signals conceal the carrierfrequency. Nyquist filtered symbols may also be used conceal the symbolclock.

In multi-antenna nodes, a separate time division duplex (TDD)transceiver may be assigned to each directional antenna. This providesfar more throughput than a single-transceiver architecture that switchesone transceiver among multiple antennas.

And finally, a distinct receive frequency may be assigned to each nodefor multiple, non-interfering transmissions in a network having amultiplicity of nodes.

A more detailed discussion of the characteristics of each node in awireless network configured to operate according to a preferredembodiment of the present invention will now be provided. Each node inthe preferred embodiment is configured to perform the following threereceiver-centric activities.

-   1. Each node continuously carries out real-time sensing and    characterization of the local spectrum at the receiver of each node    due to (potentially interfering) narrowband and wideband emitters.    For example, this analysis could use a 2K complex FFT every 100 usec    to achieve 10 KHz resolution over a 20 MHz bandwidth of interest.    These measurements will be different at each network node, since    each node has a different interference environment. Any other    information about the types of transmissions occurring in band with    potential for sharing should be gathered at this time. The    underlying principle here is that different “applications” can    tolerate differing degrees of interference (i.e. A TV broadcast may    not tolerate much interference but a wireless packet switched    network may tolerate a small percentage of packet loss).-   2. Based on the characterization performed as described above, each    node further determines a waveform with optimal Power Spectral    Density (PSD) to be used by neighboring nodes to transmit to this    node. This process consists of “inverting” the various spectral    spaces of opportunity into a realizable waveform that will approach    the optimal performance predicted by pure water filling (to be    described in detail below).-   3. Each node reports its optimal receive waveform (an economical    parametric characterization of the optimal PSD) to each of its    (e.g., handful of) network neighbors for the neighbor's use in    talking to this node, preferably along with an optimal transmit    schedule and an expiration time after which the information should    be considered stale. The waveform reporting function may be    performed in conjunction with executing other functions provided by    an optional XG transceiver application programming interface (API).    Such APIs typically include extensions of the Future Combat Systems    Communications primitives. These primitives currently support the    receiving node's power control feedback loop with each of its    neighbors. The feedback loop ensures that the XG signal arrives at    this node's receiver with the same power from every neighbor,    minimizing problems of near-far receiver masking).

This architecture is called receiver-centric, because it focuses largelyon eliminating interference, which only occurs in the nonlinear circuitsof a receiver, not at a transmitter. Each receiver is responsible forminimizing its own interference by designing a minimal-interferencewaveform, and directing all its neighbors to use this waveform totransmit to it. Different receivers will design different waveforms,depending on their local interference.

In preferred embodiments, the present invention may be characterized ashaving two different temporal metabolisms. At a low level, the localnoise interference may be estimated on a msec-by-msec time scale, makingvery short term predictions that the next msec will resemble theprevious one. On a longer time scale of tens or hundreds of msec,time-varying models of the interference may be developed based onrecognition of the type of application (e.g., video stream) causing theinterference. On this time scale, a temporal model may be used topredict which short term waveform should be used at which time.

Methods for Time-Frequency Water Filling

A description of the methods used to derive an optimal power spectrumdensity for static (short term) waveform generation based on short term(e.g., 1 msec) spectral measurements will now be provided. This methodpermits each transceiver to determine, based on measurements in itsreceiver, the power spectrum density its neighboring transceivers shoulduse to transmit to it to maximize its detected signal to noise ratio(SNR) under the constraint of limited transmit power. In the embodimentdescribed below, we assume that the interference spectrum over the nextmsec (or multiple msec) will be the same as it was in a previousmeasurement. It should be apparent to those skilled in the art, however,that when the measured spectrum changes significantly, the node mustnotify its neighbors of the updated optimal waveform.

Optimal Stationary (Short Term) Waveform Selection

The water-filling approach has long been known as the optimal way tominimize the mean squared error of a channel with colored backgroundnoise. This optimality is based on minimizing the mean squared errorbetween the signal at the detector and the originated signal (maximizingthe received SNR), subject to the constraint that the transmitted signalhas a limited total power S.

FIG. 2 shows a reference model defining the characteristics of a channel210 between a transmitter 205 and a receiver 215. In FIG. 2, P_(s)(f) isthe signal power spectrum density (Watts/Hz) at the source. H(f)represents the value of a band selection filter, which is equal to 1inside the system's desired band of operations, and 0 outside thedesired band of operations. Thus, band selection filter H(f) rejectsundesired out-of-band signals. G(f) represents the value of the optimalmatched filter. When optimal match filter G(f) is applied to the inputsignal spectrum (which includes noise) at the detector, the filteredsignal has the maximum signal to noise ratio. T is the time interval(sec) for signaling one symbol. A T of 1 usec means a signaling rate of1 M samples/sec. P_(n)(f) is the noise power spectrum density (Watts/Hz)arriving at the receiver along with the desired signal. The optimal PSDresults from the “water filling” approach, where the sum of theinterference level (weighted by the band selection filter) and theadditional received signal power from neighbors is not more than aconstant (the water level).

In a static world, the following optimal water filling would occur:

-   -   At frequencies where local interference exceeds the water level,        no power should be transmitted. This automatic transmit excision        minimizes the interference from XG signaling to legacy users.    -   At these same frequencies, the optimal matched filter in the        receiver has zero gain. This automatic receive excision        minimizes the interference from legacy users to XG users.    -   As a result, the XG network is an underlay network service that        meets the requirements for secondary use of the band, namely,        accepting any interference from existing users and agreeing to        cause no harmful interference.

In actuality, there are many reasons why this cannot happen in a realsystem. For example, there is always power leakage in the spectrabecause signals have finite bandwidth. The question is, how much isallowable, and whether the signal contained in that leakage is missedwhen it is reconstructed erroneously.

FIG. 3 graphically illustrates a method of calculating the powerspectral density (PSD) of an optimal waveform according to embodimentsof the invention. As can be seen in FIG. 3, the PSD (represented by thevariable P_(s)(f) in FIG. 3) is equal to the interference limit B minusthe measured noise floor P_(N)(f) within the band of interest. Theoptimality of the waveform is based on minimizing the mean squared errorbetween the signal at the detector and the originated signal, subject tothe constraint that the transmitted signal has a limited total power S.Receive excision suffers from finite front-end dynamic range, limitinghow large the peaks in FIG. 3 can be before they swamp the front end ofthe receiver. In this case, the spectrum cannot be inverted in one fellswoop, and may need to be passed through selective analog filteringbefore digitization.

Characterizing Background Interference

Techniques for defining the stationary waveform best tailored to theshort-term noise/interference spectrum received at a node will now bedescribed. These techniques may be used, in embodiments of theinvention, to maximize the received SNR subject to limited transmitpower. Noise in a specified bandwidth, W, appears as a bivariantGaussian signal in the in-phase (I) and Quadrature-phase (Q) channels,both measured in RMS Volts across the impedance of the measuring device,typically R=50 ohms. The total power measured in this bandwidth is thenP=(I²+Q²)/R, with a Gaussian probability density function (PDF) for bothI and Q. A change of independent variable to measured power P shows thatit has an exponential PDF equal to 1/P₀ exp (−P/P₀), with mean=standarddeviation=P₀ and median P₀ ln 2˜0.693 P₀.

Noise appears when coupling the measuring device to a resistive load oran antenna operating in a thermal environment with absolute temperatureT Kelvins. The noise power resulting from thermal excitation is k T W,where k is Boltzmann's constant 1.38*10⁻²³ Watts/Hertz/Kelvin. Thisresults from Einstein's equipartition of energy theorem, where each modeof a system in thermal equilibrium receives an excitation of kT/2joules. However, any measuring device has implementation defectscharacterized by a noise figure F, which is the ratio of its actualmeasured noise power referred to the input of the device, to the idealvalue of k T W for the thermal noise above. Here the reference value ofT is conventionally taken as 290 K (room temperature), and F is normallya few dB for low noise amplifiers. The net equivalent input noise isthus k T F W.

Attaching antennas to a node also brings in signals from intentionalradiators. When there are large numbers of uncorrelated faint signals(e.g., from many distant radiators) in the bandwidth being measured, theCentral Limit Theorem (CLT) says that the resulting summed signal againapproaches a bivariant Gaussian. Therefore, this particular backgroundinterference can also be characterized as thermal noise in a stillhotter environment, namely a higher value of T. The FCC Spectrum PolicyTask Force defines this equivalent as the interference temperatureT_(i).

Today's Spectrum: Narrowband (NB) Interferers

In addition to the noise signals described above, an antenna on a nodewill pick up strong signals from other (friendly and hostile) users.Many of these signals are narrowband (e.g., 25 KHz channels allocatedfor voice and low rate data in the UHF band), and may be non-Gaussian,since data signals often have constant envelopes for efficient poweramplification. They can appear and disappear on time scalescorresponding to users activating push-to-talk switches (i.e., a timescale of seconds). In the case of frequency hopping radios, the dwelltime in a particular 25 KHz channel may only be on the order ofmilliseconds, while a cellular phone call can last minutes. In any case,these narrow band signals appear on a spectrum analyzer as narrow“fingers” sticking up above the noise floor in the band of interest(which is normally MHz to tens of MHz wide).

The spectrum analyzer measures the power in each of a large number ofadjacent frequency bins over a short time. The shortest time intervalfor independent measurements corresponds to calculation of a discreteFourier transform (DFT), where the measurement interval is thereciprocal of the frequency bin resolution (e.g., 100 μsec measurementsfor 10 KHz resolution). The measurements described and plotted belowwere made using a Rhode & Schwartz lab quality spectrum analyzer,exporting digital data to EXCEL and MATLAB for analysis and plotting.

Probability Distribution and Statistical Analysis for “Noise Floor”Estimation

Experiments were conducted to carry out statistical characterization ofthe “noise floor” in a band of interest. The initial calibrationexperiments used a room temperature resistor (290 K) feeding alab-quality Rhode & Schwartz Model FSEM spectrum analyzer examining a 5MHz span centered on 450 MHz with 10 KHz resolution. (The spectrumanalyzer was preceded by a 24 dB (5 dB N.F.) wide band preamp). Thenoise power measured in adjacent bins was uncorrelated.

FIG. 4 shows typical sample data plotted as a histogram (relativefrequency of noise power occurrence˜PDF, versus power) resulting from500 independent power samples.

FIG. 5 shows the results of ten identical but independent measurementruns with histogram plotted on a log scale, as a function of the powermeasured. The straight line of log probability versus measured powerdemonstrates the predicted exponential probability density function 1/P₀exp(−P/P₀), with P₀=0.198 fW (−127 dBm). This measured power is 7 dBabove k T W (−134 dBm), so the experimental setup has a 7 dB noisefigure. The plot shows that the statistical fluctuation in themeasurements is far greater at higher power levels, where the smallnumber of samples results in relatively greater sampling fluctuations.

In order to estimate the “noise floor”, i.e., the interferencetemperature, N power samples were averaged for each frequency bin inorder to get a stable, unbiased estimate of average power. The PDF forthis sample mean is the N-fold convolution of the exponential PDF scaledby a factor of N, namely:

$\begin{matrix}{{{Prob}(P)} = {\frac{N^{N}}{\left( {N - 1} \right)!}\frac{P^{N - 1}}{P_{0}^{N}}{\exp\left( {{- {NP}}/P_{0}} \right)}}} & \left( {{Equation}\mspace{14mu} 1} \right)\end{matrix}$with mean P₀ (as expected) and mode (most probable power) of P₀ (N−1)/N.For N=10, the median is 0.95 P₀. The standard deviation is P₀/sqrt(N).The sqrt(N) factor in this denominator can substantially reduce thevariations (statistical fluctuations) in our estimate of the mean powerin the noise floor. If the spectrum analyzer does a DFT every 100 μsec(10 KHz resolution), averaging ten samples per bin would take 1 msec todevelop this estimate of mean power in all bins.

FIG. 6 shows a comparison of a histogram derived by averaging theresults of the 10 experiments of FIG. 5 (relative frequency ofoccurrence of a given mean power in a bin), compared with thetheoretical PDF of Equation 1 above. The agreement between the twohistograms is excellent, showing that a 10-sample power estimate in eachbin provides a good characterization of the noise floor P₀.

As described above, some frequency bins will reflect strong narrowbandsignals by measuring relatively large power. The goal is to determinewhich bins contain only noise power corresponding to the measured noisefloor (interference temperature), and which contain strong signals. TheXG signal PSD will then fill up each noise-only bin to a “water filling”temperature below the FCC-specified maximum interference temperature,and transmit zero in bins where strong signals already exceed the waterlevel. However, since all measures are probabilistic, any decision mustbe characterized by its confidence level, as described below.

The null hypothesis is that there is only noise in bin M. If the powermeasurement for this bin is statistically significant (e.g., 0.01), thenthe null hypothesis for this bin is rejected, and the conclusion is thatthere is a strong signal present. Such a large mean power measurementwould happen only in one run out of 100 if only random noise is present.This permits rejection of the null hypothesis with a fair degree ofconfidence. The significance level is the area under the tail of the PDFof the sample mean. Bins with a sample mean that exceeds thecorresponding significance threshold are judged likely (e.g., with thecorresponding 99% confidence) to contain a strong signal, and a systemaccording to the present invention transmits no power in those bins.This provides transmit excision at the transmitter to minimizeinterference to narrow band users, and receive excision in thereceiver's matched filter to minimize narrow band interference to wideband users. The remaining bins are judged likely to contain only noise,and the system transmits power in those bins up to water fillingtemperature, below the FCC-specified maximum permissible interferencetemperature.

Since the sample mean is the sum of N independent identicallydistributed variables, its distribution approaches Gaussian (CLT). The99% probability distribution threshold is P₀ (1+2.33/sqrt(N)).

FIG. 7 shows a spectrum analysis of received signals when strong narrowband signals are present. The plot in FIG. 7 was made in linear powerrather than dB in order to focus on the structure of the noise floor,and to provide an absolute reference of zero Watts. The strong signalsreach far off the top of the plot (one signal is 50 dB above the noisefloor). It is the noise floor, however, that we want to investigate.

FIG. 8 shows a histogram of received power corresponding to FIG. 7. Thehistogram (422 samples) shows the typical power distribution “bump” dueto the noise floor. Histogram samples far to the right in FIG. 8correspond to strong signals. If FIG. 8 contained all of the samples forthe received power in FIG. 7, one could observe another 78 samples (notshown in FIG. 8) located many chart widths far off to the right of thechart shown in FIG. 8 representing the strong signals. However, thesestrong signals are not of particular interest because it is the noisefloor that is being measured. Matching the low-power histogram bump witha theoretical curve corresponding to average noise power P₀ permits usto estimate the underlying noise floor P₀, in spite of the presence ofstrong signals. This will also determine the threshold at 99% confidence(at P₀ (1+2.33/sqrt(N)) to separate the bins containing the narrow bandsignals from those containing only noise. The waveform synthesized as aresult of this measurement will have support over disjoint frequencies,namely those judged to contain only noise.

Table 2 below shows measured results in characterizing a 5 MHz widenoise floor centered at 450 MHz at different times of the day (10 AM, 3PM and 6 PM). The table shows average power per 10 KHz bin.

TABLE 2 Noise floor measured as a function of time of day. Window WindowWindow Inside lab antenna antenna antenna Source 290 K Resistor antenna@ 10 AM @ 3 PM @ 6 PM P_(O) = Avg. Pwr. 2.0 * 10⁻¹⁶ W 3.5 * 10⁻¹⁶ W3.5 * 10⁻¹⁶ W 6 * 10⁻¹⁶ W 7 * 10⁻¹⁶ W

FIGS. 9 and 10 show the spectrum at 10 AM and 6 PM respectively. Thelater spectrum obviously has many more high power narrowband signals,and the effects of some of their “skirts” (unintended radiation atnearby frequencies) can be seen in the elevated noise floor. At otherfrequencies, the noise floor is elevated for unknown reasons, perhapsbecause of large numbers of additional faint signals.

Analysis of a single “noise only” bin in the time domain showed nodiscernible features. Each scan has nonrepeating wiggles (like Gaussiannoise), and successive values (at 100 used sampling interval for the 10KHz analysis bandwidth) are uncorrelated.

The results of these tests provide a strong indication that the elevated(and varying) noise floor is the sum of a large number of weaknarrowband signals (or a smaller number of wideband signals) fromunknown sources outside the building, and some skirts from strongsignals.

The results were at 450 MHz, where the noise floor was flat over the 5MHz analysis range. Examination of an equal bandwidth at centerfrequencies of 300 MHz and 39 MHz again showed a flat noise floor,although the noise floor was further elevated by a few dB.

The experiments described above with reference to FIGS. 4-10 illustrateonly one approach for characterizing the “noise floor” for water-fillingspectrum uses, based on statistical analysis of power measurements as afunction of frequency and time, and based on hypothesis testing. Afterreading the above discussion it should be apparent to those skilled inthe art that numerous other approaches may be used without departingfrom the scope of the present invention. Such experiments form the basisfor algorithms, such as the one described below with reference to FIG.15, which may be used in transceivers configured to operate inaccordance with embodiments of the present invention.

Alternative Analysis Tool: Median Filters

Instead of using the sample mean as a power estimate for each bin, asample median (nonlinear filter) may be used instead. A median filteroutputs the 50% percentile value (the median) from a vector of Nsamples. These filters are often used in image processing to eliminate“outlier” samples due to impulsive noise, while preserving sharp edges.

FIG. 11 demonstrates the use of the median power across a band at aparticular time as a normalizing factor for the power measured in eachbin. Here, for each sample spectrum, a trial median was generated acrossthe entire band. A number of the bins reflected very strong signals (farabove the band median), so bins with power more than 2.5 times themedian were discarded when calculating the final band median. For the 10AM, 3 PM, and 6 PM data, discarding these bins eliminated 71, 82, and 97of the 500 frequency bins, respectively. After rejecting considerationof these very strong signals, the band median power in remaining bandswas recalculated to characterize the noise floor by normalizing binpower with the band median power.

The theory curve gives the theoretical probability density functionobtained from integrating Equation 1. The quite similar resistor curveshows the results of measured noise from a resistor source. The otherthree curves (10 AM, 3 PM, and 6 PM) show increasing deviations from thetheoretical curve and the resistor (noise only) curve. The verticaldotted line shows the 99^(th) percentile value (P=1.6*Pmedian) for thetheoretical curve. If this value is applied as the threshold todetermine whether only noise is present in a bin or a signal is present,we will reject 1% of the bins in the noise-only case. Therefore, 99% ofthe noise only bins in the band will be targeted for reuse.

Using the same threshold algorithm at 10 AM, we would reject about 4% ofthe surviving 429 bins as containing signal. At 3 PM, we would eliminateuse of 6% of the surviving 418 bins, and at 6 PM we would eliminate 15%of the surviving 413 bins for spectrum use. The overall result is thatrespectively 82%, 79%, and 70% of the band is still available for reuseat these times.

By using the above-described method, each receiver in a networkoperating in accordance with the present invention may characterize thenoise floor of a band in terms of a median power across it, andnormalize all bin powers in terms of this measured band median.Establishing a threshold (e.g., the 99th percentile for theoretical ormeasured “pure” noise) for signal presence determines the spectrumwaveform for all neighbors to use to transmit to this node.

Representing the Waveform PSD (Quantified Bits)

The procedure above will mark a subset of the (hundreds to a fewthousand) frequency bins as available for water-filled transmissions.Since each strong signal typically covers a number of contiguous bins,run length coding is a compact and efficient representation of thissubset. If we have, for example, 12 strong signals appearing in a4096-bin spectrum analysis, this representation might appear as 12 pairsof 12-bit integers (18 bytes total). The first number of each pair isthe number of contiguous available frequency bins, and the second is thenumber of following contiguous unavailable bins. This representation ofthe desired PSD is sufficiently compact that it can easily be added tothe power control feedback loop to each neighbor node in the network,specifying both the neighbor's desired transmit power and its desiredPSD.

Tomorrow's Spectrum: Interference Temperature with Added WB Interferers

Once the FCC begins to succeed in its quest for more flexible anddynamic spectrum use, more wideband adaptive (XG-like) emitters willappear. They will use wideband signals to provide good spectrumcompatibility among independent users, and high data rates in spite oflimited transmit PSD. The resulting interference temperature will be theresult of a number of uncorrelated low power wideband signals, againsumming in the limit to a bivariant Gaussian signal. In this case, itisn't possible to identify individual emitters, just their compositepower effect. The same statistical estimation techniques described abovemay be used to estimate the interference power in each bin, and sendenough power in each bin to “fill it up” to the water-fillingtemperature below the FCC's maximum permissible interferencetemperature.

When the FCC's spectrum management revolution is complete and everyoneis using XG technology, we can expect to see white Gaussian “noise” atall frequencies everywhere, equal to the FCC's permissible interferencetemperature in each band.

Representing the XG Waveform PSD (Quantified Bits)

A compact representation of the XG PSD for this case would again be asequence of pairs of integers. The first number specifies thepermissible power to be transmitted, and the second the number ofcontiguous bins to which the value applies. For six different measurednoise floors in a band, 18 bytes can represent the desired XG PSD in thepower-and-spectrum-control feedback loop messages to neighbors. Whenstrong narrowband signals are also present, the permissible power willbe zero for that interval of bins.

Trade Studies for Design of the Short-Term Waveform

FIG. 12 depicts a model illustrating an integrated system conceptaccording to an embodiment of the present invention. As shown in FIG.12, the present invention uses water-filling (represented by thecomponent designated 1201) for each node to create its optimalshort-term receiving waveform for use by all of its neighbors tocommunicate with it. The resulting spectrum is nonuniform and coversdisjoint frequencies. As a result, the time domain version of thiswaveform (a basic symbol on the channel) will be far more complicatedthan the sin(t)/t pulse generated by a brick wall frequency domainspecification. In the following, a number of techniques for achievingvery desirable waveform characteristics are presented. These techniques,which will be described in more detail below, include using: IntersymbolInterference Mitigation (component 1202); Spread Spectrum Transmissions(component 1203); Multiple Correlators (component 1204); and Wide SenseStationary (WSS) Baseband Signals and Modulation (component 1205). Whenused in conjunction with operating nodes in a Power-Limited Regime(component 1206), these techniques provide a comprehensive spectrumreuse system that is far more efficient than any of the existing orproposed systems.

Intersymbol Interference Mitigation

The power spectrum derived from the statistical noise floorcharacterization described above is a crenelated structure withfragmented frequency support. The first challenge was to transmit asequence of basic symbols with this power spectrum (modulated to carryuser data) in a way that intersymbol interference is tolerable.Obviously, spacing the symbols far apart in time will reduce theintersymbol interference, at the cost of greatly reducing the data rate.

An alternative approach is shown in FIG. 13. Here, the desired waveformis specified as a contiguous vector of complex Fourier coefficientsX(f), with constant amplitude and some phase structure. X(f) is the setof the coefficients to be transmitted over the channel, expressed as thecomplex value of each frequency bin of a complex waveform (as wouldresult from a fast Fourier transform (FFT)). This frequency vector isthen expanded to a wider bandwidth that has zeroes in the ranges wherethe spectrum estimator has judged that interfering signals are present.This wider band frequency vector is then inverse-FFT'ed into a timewaveform for transmission across the channel. The symbol rate on thechannel is higher, corresponding to the expanded bandwidth of thisvector. Y(t) is the signal sent over the channel, which has the widerbandwidth. The undesired frequencies have been removed from y(t) by thetransmit excision.

On the receive side (toward the right-most side of FIG. 13), thereceiver does a matching FFT, “squeezes out” the undesirable frequencies(accomplishing receive excision), and then multiplies the frequencyvector by the characteristic matched filter (with phases opposite to theoriginal signal in each frequency bin) to produce the signal to bedetected at the original symbol rate. The core structure (IFFT, channel,FFT) corresponds to OFDM modulation recently commercialized in IEEE802.11a and 802.11g wireless LANs. Of course, the remaining structure (aspectrum shaping appliqué) is unique to XG. A Nyquist shaper (satisfyingthe Nyquist criterion for zero intersymbol interference) algorithmpermits finite length symbols with no intersymbol interference, in spiteof the use of noncontiguous spectrum. Thus, X(f) on the receiver side isa reconstruction of the original set of frequency coefficients with thesame name at the sender. The user symbol rate matches the bandwidth ofthis frequency domain specification X(f).

CDMA w/Multiple Correlators—White Signals

Some throughput may be lost due to spreading (processing gain). By usingmultiple code division multiple access (CDMA) correlators at eachreceiver to permit multiple neighbors to send to the node at the sametime, some of the throughput lost to spreading can be recovered.Examples of multi-correlator systems that exchange information withmultiple nodes simultaneously include GPS (receiving from up to 12satellites) and IS-95 CDMA downlink (sending to up to 64 cell phones).The present invention uses multiple waveforms (e.g., with differingphase structure) that all have the optimal PSD and have a small,tolerable level of mutual interference. For typical mobile ad hocnetworks, nodes usually have 4 to 6 neighbors, so that is a reasonableestimate of the number of correlators needed, and thus the throughputfactor that can be recovered from the spreading loss.

To achieve high performance CDMA, the present invention uses whitewaveforms (those having constant power spectrum density at the detector)with pseudorandom phase structure. A waveform of this type can be usedto provide a signal that approximates a Gaussian envelope to improveLPI/LPD, while providing orthogonal CDMA for minimal interference amongmultiple simultaneous transmissions to the destination node.

Unlike the CDMA cell towers, the present invention uses small, lighttransceivers with small antenna structures, so the transceivers willonly be able to operate in wideband half duplex, not full duplex, mode.This means that each node will alternate between a state of receivingfrom N neighbors simultaneously, and a state of transmitting either toone neighbor, or to N neighbors simultaneously when this doesn't requirean impractical dynamic range. This requires MAC protocol that is quitedifferent from conventional carrier sense multiple access/collisiondetect (CSMA/CD) protocols, such as IEEE 802.11, which uses RTS/CTSexchanges. The basic rule in the multi-correlator architecture of thepresent invention is to transmit traffic to any neighbors that aren'tcurrently transmitting. This approach provides another opportunity touse “Opportunistic Forwarding”, as described above. For extremely lowdelay traffic, packets are forwarded to any node in the direction of thedestination (lower hop count) that is not currently transmitting (like asoccer player looking for an open team mate downfield). This violatesthe customary practice of strict protocol layering, by usinginstantaneous information from the MAC layer to guide network forwardingdecisions. It minimizes the dominant end-to-end latency effect ofchannel access delay at each hop, providing far lower network latencyfor critical real time traffic.

The invention also uses orthogonal CDMA waveforms, as well as that ofthe multi-correlator MAC protocol, described above, to achieve optimalperformance.

LPI/LPD: Wide Sense Stationary Baseband Signal and Modulation

The present invention generates a waveform with the optimal PSD thatenhances LPI/LPD capability. In some applications, it is important togenerate a wide sense stationary (WSS) water-filling waveform thatminimizes features susceptible to intercept by a second ordercyclo-stationary interceptor. Wide sense stationarity requires that allsecond order statistics of the signal are constant functions of time,meaning that the expected value of the signal (mean) is constant (chosento be zero), and the expected variance (mean power) and theautocorrelation function are also constant functions of time.Specialized filtering and linear modulation architecture may be used toeliminate any spectral redundancy (nonlinear constant envelope andcontinuous phase modulations are easily susceptible to second ordercyclostationary intercept). This symbol generation and modulationstructure involves Nyquist filtering of the symbols (limiting symbolfrequencies to 1/(2T), where T is the symbol period on the channel), toproduce constant mean power signaling, and to prevent exposure of thesymbol clock by a magnitude-square operation.

In addition, the symbols are modulated onto a carrier using uncorrelatedI and Q channels with equal mean power to prevent exposure of thedoubled carrier frequency by a squaring operator. Making I and Quncorrelated may be accomplished either by sending uncorrelated data oneach channel, or by making Q the Hilbert transform of I, resulting insingle sideband modulation of the carrier. Hilbert transforms aredescribed in A. Papoulis, Probability, Random Variables, and StochasticProcesses, McGraw Hill 1991, which is incorporated herein by referencein its entirety.

Use of Power Limited Signals for Spectrum Compatibility and LPI/LPD:(Data Rate<<Bandwidth, EB˜NO)

In preferred embodiments of the present invention, network links areoperated in the power limited regime (transmitting far less than one bpsper Hz), rather than the conventional bandwidth limited regime (withmore than one bps per Hz), which requires high SNR (e.g. >10 dB for 10-5BER with QPSK). At the cost of lower data rates, this capability, alongwith closed loop power control on each link, will permit an entirenetwork to operate in LPI/LPD mode. Using wide bandwidths and low poweralso greatly enhances the spectrum compatibility among multiple XG usersand legacy users.

Preferably, the optimal water-filling waveforms produced by theabove-described methods are used in a spread spectrum mode, where theuser bit rate is roughly an order of magnitude less than the basicsymbol rate. Spectrum spreading improves LPI/LPD and interferenceperformance by a factor of the processing gain, at the cost of a reduceduser data rate. The recently introduced Turbo-Hadamard codes, whichprovide very low rate (e.g. rate ⅛ or 1/16) forward error correction(FEC) coding, may be used to address this problem. A rate of ⅛ or 1/16means that one user bit turns into 8 or 16 bits with forward errorcorrection on the channel. These codes have been demonstrated to operatewith 10-5 BER down to Eb/No ratios below 0 dB at the detector, which isclose to the ultimate Shannon bound of Eb/No=−1.6 dB. With thisapproach, spectrum spreading and FEC are combined into a singleoperation. Operation at such low Eb/No ratios provides LPI/LPD andexcellent spectrum compatibility with other users. The spread spectrum(e.g., an order of magnitude greater bandwidth than the data rate)permits resolving multipath components in the time domain, perhaps evenpower combining the energy of the different multipath components in arake receiver.

FIG. 14 shows a high-level block diagram of a network node 1400 (such asa software radio, a router, etc.) configured to operate according toembodiments of the present invention. As shown in FIG. 14, node 1400comprises a receiving antenna 1405, a transmitting antenna 1410, areceiver 1415, an input filter 1480, multiple correlators 1485, anetwork data processor 1490, a MAC controller 1495, a spectrum analyzer1420, a buffer 1425, a signal data processor 1430, a transmitter 1450, acompressor 1445, an output filter 1440 and a waveform profile generator1435.

Receiver 1415 continuously monitors and receives aggregate signals fromthe networking environment. When a signal is received, receiver 1415passes the signal to spectrum analyzer 1420, which is configured tocontinuously measure power in all the frequencies contained in afrequency band of interest. The output from spectrum analyzer 1420 is aseries of power values as a function of frequency. Typically, althoughnot necessarily, spectrum analyzer 1420 would store multiple powerfunctions in a buffer 1425, which is coupled to a signal data processor1430 configured to create a model or “normalized” power function, suchas by computing the average or median power reading at each frequency inthe band of interest over a given period of time. Signal data processor1430 conveys the model power function to waveform profile generator1435, which, in preferred embodiments, is configured to subtract themodel power function from a specified interference temperature, limit orthreshold, in order generate an optimal waveform profile that specifieswhich frequency neighboring nodes should use to communicate with node1400. More particularly, waveform profile generator 1435 produces anoptimal waveform profile 1401 that requires neighboring nodescommunicating with node 1400 to use zero power at frequencies node 1400has determined are already populated with signals generated by legacycommunication devices in the network.

Output filter 1440 conforms the optimal waveform profile created bywaveform profile generator 1435 to a format that will be understood byneighboring node 1460. In other words, output filter 1440 performstransmit excision to remove power from certain frequencies thatneighboring node 1460 has determined should not be used because thosefrequencies are carrying data signals from primary users (e.g., legacyand high-priority users, such as police, governmental or militaryentities). Optionally, node 1400 includes a compressor 1445, which maybe implemented in hardware, software, or both, which compresses theoptimal waveform profile 1401 prior to sending it to neighboring node1460 via transmitter 1450 and antenna 1410. In a preferred embodiment,node 1400 would also provide feedback information to neighboring node1460 to assure that the signal neighboring node 1460 sends back to node1400 arrives with the same power level as that of every other neighbor,minimizing problems of near-far receiver masking.

Neighboring node 1460 typically comprises substantially the samecomponents as node 1400. For simplicity's sake, however, neighboringnode 1460 is shown in FIG. 14 as being comprised of a processor 1467,decompressor 1465, output filter 1470, receiving antenna 1455 andtransmitting antenna 1475. Neighboring node 1460 receives the signalcontaining the optimal waveform profile 1401 from node 1400 via antenna1455. If the signal containing optimal waveform profile 1401 iscompressed, decompressor 1465 of neighboring node 1460 decompresses thesignal before passing it along to processor 1467. Processor 1467 isconfigured, in conjunction with output filter 1470, to use the optimalwaveform profile 1401 to generate and send a new signal (optimalwaveform 1402) conforming to the optimal waveform profile 1401 whentransmitting data to node 1400. In other words, the signal has beenfiltered such that there is no power transmitted at the frequencieswaveform profile generator 1435 determined carried signals from legacytransmission devices.

Neighboring node 1460 transmits information back to node 1400 in theform of the optimal waveform 1402 via transmitting antenna 1475.Notably, nodes 1400 and neighboring node 1460 may or may not use asingle antenna to perform both the transmit and receive functions.During this transmission, optimal waveform 1402 may be influenced and/orpartially contaminated by interference 1412 produced by other emitters(e.g., legacy transmitting devices) existing in the wireless networkingenvironment.

When the signal carrying optimal waveform 1402 (combined with noise andinterference produced from interference 1412) is received at node 1400via antenna 1405 and receiver 1415, it will again be passed to spectrumanalyzer 1420 for analysis and generation of an “updated” optimalwaveform profile that takes into account changes in the wirelesscommunication environment that may have occurred since the last optimalwaveform profile was generated. Such changes in the wirelesscommunication environment might occur, for example, due to a change inthe physical location of node 1400 or other mobile nodes in the network,or by new emitters coming online in the vicinity of node 1400.

Receiver 1415 also passes the incoming signal to input filter 1480(preferably, an optimal matched filter), which is configured to zero out(or ignore) any power carried at frequencies node 1400 previouslydetermined were occupied by signals produced by legacy systems. In otherwords, input filter 1480 performs receiver excision because it isconfigured to be most sensitive where the optimal waveform profile 1401requires there to be power, and completely insensitive to power signalscarried on frequencies where the optimal waveform profile 1401 for node1400 requires there to be no signal. In this way, input filter 1480removes the noise and interference caused by interference 1412.

Input filter 1480 passes the filtered signal to a set of multiplecorrelators 1485 (containing correlators C1 through CN), each of whichis configured to respond to unique patterns associated with one or moreneighboring nodes in the vicinity of node 1400. Using multiplecorrelators in this manner allows node 1400 to receive from multipleneighboring nodes simultaneously. Once the incoming signal has beenfiltered by input filter 1480 and its source has been identified bymultiple correlators 1485, it is then passed on to network dataprocessor 1490 for processing of the data carried by the signal,according to the specific requirements, applications and/or networkingprotocols associated with node 1400 and the wireless communicationsnetwork in general.

As discussed above, node 1400 is configured to operate in half-duplexmode, meaning that it is always in either transmit mode or receive mode,but not both at the same time. Accordingly, node 1400 also includes amedia access controller (designated MAC controller 1495 in FIG. 14),which is responsible for switching node 1400 back and forth betweenreceive mode and transmit mode. When node 1400 is operating in receivemode, it is capable of receiving multiple signals from multipleneighboring nodes, each of which uses the optimal waveform 1402, asdefined by node 1400 and transmitted to those neighboring nodes. Node1400 identifies the source of those signals using correlators C1 throughCN (designated 1485 in FIG. 14).

In alternative embodiments, node 1400 may be configured to use a common,low data rate configuration channel to transmit waveform profile 1401 toneighboring node 1460.

Although the example node shown in FIG. 14 and discussed in detail aboveshows components, such as processors, controllers, generators, antennas,filters and buffers, as separate physical components residing at node1400, those skilled in the art would recognize and appreciate the factthat the invention may be advantageously combined or divided, dependingon the needs of the particular implementation, into more or fewerprocessors, controllers, generators, antennas, filters and buffers thanthe number shown in the embodiment of FIG. 14, and that such componentsdo not necessarily have to reside at each of the nodes in the network.It should be apparent, for example, that, although the example node inFIG. 14 shows two separate antennas for clarity, network nodes typicallyuse only a single antenna for both transmit and receive modes.Accordingly, such alternative configurations are not meant to falloutside the scope of the claimed invention.

FIG. 15 contains a high-level flow diagram illustrating the stepsperformed in an embodiment of the present invention in order to processsignals and manage interference received at a node, such as node 1400described above with reference to FIG. 14. Beginning at step 1505, thenode receives an aggregate radio signal available at the receiving node.When a signal is received, the system passes the signal through twoparallel processing chains.

The first processing chain, depicted on the left side of FIG. 15,comprises steps 1515, 1520, 1525, 1530 and 1535. In step 1515, aplurality of instantaneous power measurements over a given length oftime are acquired for each frequency in a band of interest and, in someembodiments, stored in a buffer as shown in FIG. 14. Next, at step 1520,the system determines a model power level based on the plurality ofpower levels acquired in step 1515. Such model power level may bedetermined, for example, by calculating the average or median powerlevel at each frequency during the period of time. The model power levelat each frequency is then subtracted from an interference limit, step1525. The interference limit may be specified by a government agency, arule-making body an industry standard and/or some combination of all ofthem.

The difference between the interference limit and the model power level,which represents the amount of power spectrum available for use by nextgeneration systems, is used, at step 1530, to generate an optimalwaveform profile (OWP) for the receiving node to receive transmissionsfrom a neighboring node. The optimal waveform profile requires thatcertain frequencies will not be used to communicate with this nodebecause using those frequencies will interfere with legacy and primaryusers communicating in the band of interest. At step 1535, the optimalwaveform profile (OWP) is transmitted to other nodes in the network. Atthis point, control returns to step 1505 where the system again receivesthe aggregate signal available at the receiving node.

While the optimal waveform profile (OWP) is being calculated, generatedand reported to other nodes in steps 1525, 1530 and 1535, the receivedaggregate signal is also examined in a parallel chain of steps (steps1540, 1545, 1550 and 1555 in FIG. 15) to determine whether the signalcontains data sent specifically to this node from another specific nodein the network. At step 1540, for instance, the aggregate signal isfiltered, such as by an optimal matched filter, to remove all powercarried on frequencies corresponding to transmissions by legacy andprimary users. These frequencies are the same frequencies specified foravoidance by the optimal waveform profile (OWP) generated in step 1530.Next, at step 1545, the filtered signal is correlated to identify asource for the transmission by determining whether it contains a uniquepattern associated with a particular node in the network.

If a particular node is identified as the source of the transmitted andreceived signal, the system may optionally instruct that node to adjustthe power level on its transmission (step 1550) so as not to overwhelmother nodes in the immediate vicinity. At step 1555, the signal data ispassed to a network processor for further processing in accordance withthe requirements of various applications and protocols, and controlpasses, once again, back to step 1505, where the system continues toreceive an aggregate signal.

Clustering of Nodes to Manage the Large Dynamic Range of RF Links in aMobile Wireless network.

Typical mobile networks have RF links that have a path loss (PL) rangeof 60-120 dB. This corresponds to a line-of-sight range variation (withr² loss) of 1000 to 1, or ground mobile range variations (with r⁴ loss)of about 30 to 1. Co-site interference limits overall networkperformance because (high power) transmission to a distant receiver“deafens” all nearby receivers. This 60 dB dynamic range of path loss isbeyond the normal power control and spurious-free dynamic range (SFDR)capabilities of typical RF transceivers.

In a preferred embodiment of the present invention, co-site interferencemay be addressed by dividing the available spectrum into two bands, withsimultaneously operating, non-interfering half duplex transceivers oneach. One set of transceivers is used within automatically formedshort-range clusters, e.g., nodes with 60-90 dB PL to their neighborswithin a cluster (intra-cluster communications). The other set oftransceivers are configured to operate over considerably longer paths(PLs in the 90-120 dB range), using far greater transmitter power, toprovide inter-cluster communications. Factoring the spectrum into twobands with greatly differing path loss capabilities is one way ofreducing the dynamic range required of any single transceiver to a rangethat is much easier to manage (e.g., 30 dB).

A manager for each cluster is automatically selected, e.g., by using aK-means (position vector quantization) algorithm. The manager provides apoint coordination function that controls use of any inter-cluster link.The manager is accessed over the intra-cluster links, and grants onlyserialized access to use an inter-cluster link. Traffic need not flowthrough the manager, only the channel access permission for use of theinter-cluster links. This eliminates co-site interference within eachcluster, by assuring that, at most, one long range transceiver istransmitting at a time within each cluster.

Multiple CDMA correlators may be used in each short-range transceiver,and a different CDMA transmission code assigned for each neighbor. Thus,a node can receive from multiple neighbors at the same time, as long asit is not transmitting. The ability to receive from multiple neighborssimultaneously on each band recovers some of the bandwidth lost to theCDMA code's spectrum spreading. Simultaneous CDMA reception frommultiple neighbors is enhanced by using closed loop power control fromeach neighbor (e.g., over a 30 dB dynamic range) to assure that equalpower arrives at the receiver from each neighbor.

In preferred embodiments, each correlator provides “carrier sense” toindicate when a particular neighbor is transmitting. The node alternatesbetween modes of transmitting to a single neighbor, or receiving frommultiple neighbors. When no outgoing traffic is queued to a neighborthat is not currently transmitting, the node switches to receiving mode.When outgoing traffic is queued and no neighbor is transmitting to thisnode, the node switches to transmit mode. The same type ofmulti-correlator operation may be used for the long-range transceiversto improve their aggregate reception data rates as well.

Co-site interference may also be minimized by giving each node its ownreceive frequency. Under this scheme, transmit frequency is changed on apacket-by-packet basis to send each packet to the appropriate neighbornode on the neighboring node's own receive frequency. This permits anode to transmit out of one directional antenna (on the appropriatetransmit frequency, i.e., the neighbor's receive frequency), whilesimultaneously receiving through multiple other directional antennas,pointed in other directions, on this node's own receive frequency. Thebenefits of simultaneous transmit and receive include both highernetwork throughput, and lower end-to-end latency for real time traffic.

A problem arises, however, when there are a multiplicity of nodes in anetwork and each node uses directional antennas to transmit and receiveat the same time. Suppose, for example, the node comprises a movingvehicle equipped with multiple directional antennas, one on each side ofthe vehicle. The directional antennas usually do not provide enoughsignal separation to use the same frequency to broadcast on one side ofthe vehicle while receiving on the opposite side. The transmittingantenna will simply overwhelm the receiving antenna. For this reason,the transceivers on each node (or, in this case, each vehicle) in thenetwork may be configured to have its own unique receive frequency andalternate between transmit and receive modes. While the node is inreceive mode, and not transmitting, it will pick up any signal on itsassigned frequency.

In some networks, there is simply not enough spectrum available toallocate a unique receive frequency to every node in the network. Thepresent invention addresses this problem by assigning a small number ofreceive frequencies to nodes in such a way that nodes using the samereceive frequency are far enough apart so that they do not pick up eachother's transmissions. In other words, adjacent nodes are never assignedthe same frequency.

FIGS. 16, 17 and 18 show the results of simulation experiments forassigning different frequencies (sometimes referred to as “coloring”) arandom layout of fifty nodes in a network. First, as shown in FIG. 16,an attempt was made to assign frequencies to all 50 nodes using only twofrequencies. In FIG. 16, the first frequency is represented by thesquares and the second frequency is represented by the circles. If alink connects two nodes having the same frequency (shape), then thatlink is deemed to be unusable because it means those two nodes aresharing the same receive frequency. As FIG. 16 shows, it is not possibleto assign one of the two frequencies (i.e., one of the two shapes) toall 50 nodes in such a way that there are no links connecting the sameshape. Links connecting the same shape are shown as dotted lines in FIG.16. So, for example, as shown in FIG. 16, the links between nodes 1 and7, nodes 7 and 11, nodes 11 and 21, nodes 11 and 12, nodes 13 and 3,etc., are all unusable links. In this case, only 55% of the links inFIG. 16 are usable.

FIG. 17 shows the results of attempting to assign receive frequencies tothe same set of nodes using three different frequencies (shapes) insteadof two. Thus, a new frequency (represented by the star shape in FIG. 17)is available to use at different nodes as assigned receive frequencies.As can be seen in FIG. 17, after all of the nodes have been assigned oneof the three available receive frequencies (shape), more of the links(89%) are usable than occurred with two frequencies (shapes). However,there are still a number of unusable links between nodes having the samefrequencies (e.g., links connecting nodes 1 and 7, nodes 4 and 5, nodes12 and 13, etc).

FIG. 18 shows the results of attempting to assign receive frequencies(shapes) to the same set of 50 nodes using four frequencies (shapes)instead of three. Thus, a fourth frequency (represented in FIG. 18 bythe triangle shape) is now available to assign to nodes. Having fourdifferent frequencies (i.e., four different shapes) to assign to thefifty nodes makes it possible to have no adjacent nodes with the samefrequency (or same shape). Thus, 100% of the nodes are connected byusable links. This being the case, there should be enough physicalseparation between nodes using the same receive frequency to transmitand receive simultaneously without experiencing harmful interference.

FIG. 19 shows a high-level flow diagram illustrating the steps that maybe performed by a processor configured to assign receive frequencies toa multiplicity of nodes in accordance with the present invention, toensure that there are no adjacent nodes that use the same receivefrequency. Links that are connected to nodes having the same receivefrequency are unusable. As shown in FIG. 19, the first steps, step 1910and 1920, are to select a first receive frequency and assign it to allof the nodes in the network. Next, a new frequency (receive frequencynumber two) is allocated, step 1930, and made the current frequency(using the variable “curr_frequency”) at step 1940.

Next, in step 1950, a node that is at one end of an unusable link andthat is not already using the current frequency is selected. The currentfrequency is assigned as the receive frequency for the selected nodeunless doing so would create another unusable link in the network. Seestep 1960. In the next step, step 1970, the system checks to see ifthere are any more unusable links. If the answer is no, processing stopsbecause all links are now usable and there is no need to proceed anyfurther. However, if it is determined at step 1970 that unusable linksstill exist, then the next step, step 1980, is to determine whether anyof the unusable links are connected to a node not already using thecurrent color. If the answer is yes, then processing returns to steps1950 and then 1960, wherein that node is selected and assigned thecurrent frequency unless doing so would create another unusable link.If, on the other hand, it is determined at step 1980 that none of theunusable links are connected to a node not using the current frequency,then processing returns again to step 1930, where another frequency mustbe allocated and then assigned to nodes connected to unusable linksuntil either all links become usable or no other nodes can be assignedthe newly allocated frequency.

The above-described embodiments are by no means meant to limit the scopeof the invention. Though the invention has been described with respectto preferred embodiments thereof, many variations and modifications willbecome apparent to those skilled in the art upon reading this disclosureand the following claims, as well as practicing the claimed invention.It is therefore the intention that the claims be interpreted as broadlyas possible in view of the prior art, to include all such variations andmodifications.

1. A method of managing interference in a radio communications network,the radio communications network having a multiplicity of nodes, eachnode in the multiplicity having attached thereto a set of antennas, eachantenna in the set being oriented in a unique direction relative toother antennas attached to each the node, the method comprising thesteps of: dividing the multiplicity of nodes into a plurality of nodeclusters; assigning a unique receiving frequency to each node in atleast one node cluster; assigning a unique transmission frequency toeach antenna in the set of antennas attached to a first node in the atleast one node cluster; wherein the unique transmission frequencyassigned to the each antenna corresponds to the unique receivingfrequency assigned to a neighboring node in the at least one nodecluster; and wherein the neighboring node is located in the uniquedirection in which each the antenna is oriented to face.
 2. The methodof claim 1, further comprising the step of transmitting an outgoingmessage from the first node to the neighboring node via a transmittingantenna in the set of antennas attached to the first node using theunique receiving frequency assigned to the neighboring node.
 3. Themethod of claim 2, further comprising the step of transmitting a secondoutgoing message from the first node to another neighboring node in theat least one node cluster via a second transmitting antenna in the setof antennas attached to the first node using the unique receivingfrequency assigned to the another neighboring node.
 4. The method ofclaim 3, wherein the step of transmitting the outgoing message to theneighboring node and the step of transmitting the second outgoingmessage to the another neighboring node are carried out simultaneously.5. The method of claim 2, further comprising the step of receiving, atthe first node, via a second antenna in the set of antennas attached tothe first node, an incoming message from another neighboring node in theat least one node cluster, using the unique receiving frequency assignedto the first node.
 6. The method of claim 5, wherein the step oftransmitting the outgoing message to the neighboring node and the stepof receiving the incoming message from the another neighboring node arecarried out simultaneously.